1. Field of the Invention
The present invention relates to vapor etching of samples and, more particularly, to vapor etching of semiconductor materials.
2. Description of Related Art
Vapor etching of semiconductor materials and/or substrates is accomplished using gases such as xenon difluoride. Specifically, in xenon difluoride etching, the xenon difluoride gas reacts with solid materials, such as silicon and molybdenum, such that the materials are converted to a gas phase. This removal of these materials is known as etching.
Typically, with gas phased etching, especially in case of xenon difluoride etching, the reaction is exothermic or creates heat. This heat will cause a temperature increase on the part being etched. The part being etched is referred to as the sample. Increased temperature can influence critical parameters such as etching rate, or speed of etching, and selectivity, which is the relative rate that the material desired to be etched etches relative to a material that is desired to remain. Also, increased temperatures can lead to problems such as destroying sensitive materials, such as polymers.
A common approach to xenon difluoride etching is through the pulsed method of etching. In this mode, xenon difluoride is sublimated from a solid to a gas in an intermediate chamber, referred to as an expansion chamber, which can then be mixed with other gases. The gas(es) in the expansion chamber can then flow into an etching chamber to etch the sample, referred to as the etching step. The main chamber is then emptied through a vacuum pump. This cycle, including the etching step, is referred to as an etching cycle. These cycles are repeated as necessary to achieve the desired amount of etching.
It should be noted that any heat that occurs during the etching will raise the temperature of the sample. By simply evacuating the chamber at the end of the etching cycle and repeating the cycle, there is little opportunity for the sample to return to its original temperature. This is in part due to the fact that the evacuation of the chamber, which by definition is reducing the number of gas molecules inside of the chamber, reduces the thermal conductivity of the gases inside of the chamber. Therefore, generally, each etching cycle will cause the sample to continually increase in temperature. It has been observed that increased temperature has resulted in reduced selectivity of silicon versus low-pressure chemical vapor deposited silicon nitride. Silicon nitride is a very common material in semiconductor or microelectromechanical systems devices and, in most cases, minimizing attack on silicon nitride when etching silicon is highly desirable.
Reduced selectivity of silicon versus other materials, including silicon nitride has also been attributed to the presence of the products of the etching process. These products of the reaction, such as silicon tetrafluoride, attack the non silicon materials reducing the selectivity.